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SMT assembly process
SMT assembly process is closely related to each process step before welding, including capital investment, PCB design, component solderability, assembly operation, flux selection, temperature/time control, solder and crystal structure.

1 Solder

At present, the most commonly used solder for wave soldering is eutectic tin-lead alloy: tin 63%; lead 37%, solder temperature in the solder pot should always be known, the temperature should be higher than the alloy liquid temperature of 183 °C, and the temperature is uniform. In the past, solder pot temperatures of 250°C were considered "standard."

With the innovation of flux technology, the uniformity of the solder temperature in the entire solder pot has been controlled, and a preheater has been added. The trend is to use a solder pot with a lower temperature. It is common to set solder pot temperatures in the 230-240°C range. In general, the components do not have a uniform thermal mass and it is necessary to ensure that all solder joints reach a sufficient temperature in order to form a qualified solder joint. The important issue is to provide enough heat to increase the temperature of all leads and pads to ensure the solder flowability and wet both sides of the solder joint. The lower temperature of the solder will reduce the thermal shock to the components and the substrate, which will help reduce the formation of scum. At lower strengths, under the combined action of the flux coating operation and the flux compound, the wave crest outlet can have Adequate flux, which can reduce the generation of burrs and solder balls.

Solder compositions in solder pots are closely related to time, ie change over time, which leads to the formation of scum, which is the reason for the removal of residues and other metal impurities from the soldered components and in the soldering process. The cause of tin loss. These factors can reduce the solder fluidity. In the procurement, the maximum limit of the tin content of the metal trace dross and the solder to be specified is in each standard (as specified in IPC/J-STD-006). In the soldering process, the requirements for solder purity are also specified in the ANSI/J-STD-001B standard. In addition to the restrictions on scum, 63% tin; 37% of the lead alloy in the minimum tin content of not less than 61.5%. Gold concentration in the gold and organic bathing layers on the wave soldered assembly is faster than in the past. This build-up, combined with significant tin loss, can cause the solder to lose fluidity and create soldering problems. Rough, granular solder joints are often caused by scum in the solder. The dull, coarse grain solder joints due to the accumulated scum in the solder pot or the component itself may also be a sign of low tin content, not a local special solder joint, or the result of tin loss in the tin pot. This appearance may also be due to vibration or shock during solidification.

The appearance of solder joints can directly reflect process problems or material issues. It is important to check the solder pot analysis in order to maintain the solder "full pot" condition and follow the process control scheme. Soldering in the solder pot due to scum in the solder pot is generally not necessary, as the solder in the solder pot is always full due to the requirement to add solder to the solder pot in conventional applications. . In the case of tin loss, the addition of pure tin helps maintain the desired concentration. In order to monitor the compounds in the tin pot, routine analysis should be performed. If tin is added, it should be sampled and analyzed to ensure that the solder composition is correct. Excessive scum is another thorny issue. There is no doubt that scum is always present in the solder pot, especially when soldering in the atmosphere. The use of "chip crests" to solder high density components is helpful because the surface of the solder exposed to the atmosphere is too large to oxidize the solder, resulting in more dross. The surface of the solder in the solder pot is covered with a scum layer, and the oxidation rate is slowed down.

In welding, more dross is generated due to the turbulence and flow of the peaks in the tin pan. The recommended method is to remove the scum. If boring is done frequently, more scum will be produced and more solder will be consumed. Dross may also be trapped in the wave crest, resulting in wave crest instability or turbulence, thus requiring more maintenance of the liquid composition in the solder pot. If it is allowed to reduce the amount of solder in the solder pot, scum on the solder surface will enter the pump, and this phenomenon is likely to occur. In some cases, particulate solder joints may contain floating particles. The initially found scum may be caused by a rough crest and may clog the pump. Tin pots should be equipped with adjustable low-capacity solder sensors and alarm devices.

2 Peaks

In the wave soldering process, the crest is the core. The preheated, flux-coated, non-contaminant metal can be sent to the welding station by a conveyor belt, contacted with a solder having a certain temperature, and then heated, so that the solder reacts chemically, and the solder alloy forms interconnects by peak power. Is the most critical step. At present, the commonly used symmetrical peaks are called main peaks, and the pump speed, peak height, depth of penetration, transmission angle, and transmission speed are set to provide all-around conditions for achieving good welding characteristics. The data should be properly adjusted, and the solder operation should be decelerated and slowly stopped after leaving the peak (outlet). The PCB will eventually push the solder to the outlet as the wave runs. In the worst case scenario, the surface tension of the solder and the optimized board peak operation allow zero relative movement between the component and the outlet peaks. This husking zone is achieved by removing the solder from the board. Sufficient inclination shall be provided without defects such as bridging, burrs, wire drawing and solder balls. In some cases, the outlet of the crest needs to have a hot air flow to ensure that possible bridging is eliminated. After surface mount components are mounted on the bottom of the board, the flux chip or bubbles in the “quench peak” area formed later are sometimes compensated for before the peak leveling is performed using the turbulence chip peaks. The high vertical velocity of the turbulent wave crests helps to ensure solder contact with the leads or pads. The vibrating portion behind the flattened laminar wave crests can also be used to eliminate bubbles and ensure that the solder achieves satisfactory contact with the components. The welding station basically should be: high-purity solder (standard), peak temperature (230 ~ 250 °C), the total time of contact peak (3 ~ 5 seconds), the depth of the printed board immersed in the peak (50 ~ 80 %) to achieve a parallel transfer path and flux content in the tin pan in parallel with the peak and track.

3 Wave cooling after soldering

A cooling station is usually added to the tail of the wave soldering machine. In order to limit the tendency of copper-tin intermetallic compounds to form solder joints, another reason is to accelerate the cooling of the components and to prevent the board from shifting when the solder is not completely cured. Cool the module quickly to limit exposure of the sensor to high temperatures. However, the danger of thermal shock to components and solder joints due to aggressive cooling systems should be taken into account. A well-controlled "gentle and stable", forced air cooling system should not damage most components. There are two reasons to use this system: The plates can be handled quickly without holding by hand, and the temperature of the components can be kept below the temperature of the cleaning solution. People are concerned about the latter reason, which may be the cause of foaming of certain flux residues. Another phenomenon is the phenomenon of reaction with certain flux scums, which causes the residue to “wash out”. There is no fixed formula to meet these requirements in ensuring that the data set by the welding station satisfies all machines, all designs, all materials used, and process material conditions and requirements. It is necessary to understand each step of the entire process. 4 Conclusion In conclusion, in order to obtain the best welding quality and meet the needs of users, each process step before welding and welding must be controlled, because each step of the entire SMT assembly process is interrelated and interacts with each other. The problems will be within the overall reliability and quality. The same is true for welding operations, so all parameters, time/temperature, amount of solder, flux composition, and transfer speed should be strictly controlled. For defects generated during welding, the cause should be identified as soon as possible, analyzed, and appropriate measures taken to eliminate various defects that affect quality in the bud. In this way, we can ensure that the products we produce meet the technical specifications.